Night vision devices are well known in the art. Such devices are used by military and law enforcement personnel to enhance a user's ability to see during operations conducted in low light conditions. Generally, these devices operate in the 0.4 to 1.0 .mu.m region of the electromagnetic spectrum and include an objective lens assembly, an ocular lens assembly and image intensifier tube. Image intensifier tubes multiply the amount of incident light received and thus provide an increase in light output which may then be supplied to a camera or directly to the eyes of a user. In regard to image intensifier tubes, reference is made to U.S. Pat. No. 5,023,511 entitled OPTICAL ELEMENT OUTPUT FOR AN IMAGE INTENSIFIER DEVICE which issued to Earle N. Phillips on Jun. 11, 1991 and U.S. Pat. No. 5,077,611 entitled UNIVERSAL IMAGE INTENSIFIER TUBE which issued to Phillips et al. on Dec. 31, 1991, both of which are assigned to ITT Corporation, the assignee herein.
There are various types of night vision devices. One type of night vision device is the night vision goggle system. This system is used at night by individuals to enhance their ability to perform tasks such as walking, driving, observation, map reading and others. In this regard, reference is made to U.S. Pat. No. 3,781,560 entitled NIGHT VIEWING SYSTEM FOR READING WITHOUT LIGHT which issued to Deburgh et al. on Dec. 25, 1973, U.S. Pat. No. 4,266,129 entitled DEVICE FOR VIEWING RESIDUAL LIGHT IN THE VISIBLE AND THE NEAR INFRARED SPECTRUM which issued to Versteeg et al. on May 5, 1981 and assigned to N. V. Optische Industrie "De Oude Delft", Delft, Netherlands and U.S. Pat. No. 4,463,252 entitled NIGHT VISION GOGGLE SYSTEM which issued to Brennan et al. on Jul. 31, 1984 and assigned to Baird Corporation.
Two types of night vision goggle systems are the AN/PVS-7A and AN/PVS-7B. In these systems, the ocular lens assembly further includes a collimator lens assembly and dual relay lens assembly. The collimator lens assembly is positioned to provide a collimated output image of the image intensifier output screen. The relay lens assembly then relays the collimated image from the collimator lens assembly to the user's eye for viewing. Commercial versions of these systems have been converted into night vision binoculars by the addition of longer focal length objective lenses to provide greater magnification. In addition, ITT Electro Optical Products Division of ITT Corporation in Roanoke, Va. has manufactured night vision binoculars for the U.S. military having 3X and 4.5X magnification. Many of these devices utilize GEN II or GEN III image intensifier tubes, which are well known in the industry.
In another type of night vision device, the ocular lens assembly is replaced by a relay lens so as to enable photographic recording of images. Alternately, the standard objective lens may be replaced with a larger objective lens to enable use of the device as a long range observation system or weaponsight. In regard to weaponsights, reference is made to copending U.S. patent application Ser. No. 07/647,544 entitled NIGHT SIGHT FOR MISSILE LAUNCHER by Phillips et al., filed on Jan. 29, 1991 and U.S. Pat. No. 5,084,780 entitled TELESCOPIC SIGHT FOR DAY/NIGHT VIEWING which issued to Earle N. Phillips on Jan. 28, 1992, both of which are assigned to ITT Corporation.
Moreover, a night vision device may be configured as a remote viewer by utilizing a camera or charge coupled device (CCD) type video device in place of the ocular lens assembly. This configuration provides image recording and remote viewing capabilities. In addition, night devices have been configured as gunner's and commander's sights for armored vehicles as well as driver's viewers. In this regard, reference is made to U.S. Pat. No. 5,029,963 entitled REPLACEMENT DEVICE FOR A DRIVER'S VIEWER which issued to Naselli et al. on Jul. 9, 1991 and is also assigned to ITT Corporation.
As can be ascertained, it is desirable to utilize a navigation aid for many military and law enforcement operations. One such navigation aid is the standard lensatic compass. A standard compass includes an angular scale that is used to align the compass with a distant object in order to obtain a "bearing". A disadvantage with utilizing such a compass at night in conjunction with a night vision device is that the scale may be very difficult to read due to lack of illumination. In addition, the scale may be difficult to read because of insufficient contrast between the scale and the background to which it is affixed.
Compass scales which are illuminated in order to aid reading of the scale at night are well known in the art. By way of example, Tasco Corporation manufactures binoculars designated as models 322 BCW and 322 BW each of which utilize an illuminated compass scale that is unistructural with the binocular. The illumination of the scale in each binocular is controlled by a switch which enables a user to turn the illumination on or off as desired. During nighttime operations, the user locates and focuses on a desired object in the normal manner with the illumination off. The user then illuminates the scale by manipulating the switch which enables the user to read the scale and determine direction.
However, such binoculars are designed for use with the unaided eye and without a night vision device. Therefore, a relatively high level of light is used to illuminate the scale in such binoculars which is sufficient to enable the unaided eye to read the scale. It is well known that such a level of light causes a degradation of the resolution of the image intensifier tube in a night vision device. Consequently, the use of such prior art illuminated scales in a night vision device would result in a degradation of resolution and a washed out effect of the viewed scene. As such, the ability of a user to read the compass scale after aligning the compass with a distant object would be severely restricted due to the washed out effect caused by prior art illuminated compass scales.
Therefore, there is a need in the art for a compass module having an illuminated compass scale with a sufficient illumination level to enable an operator to read the scale without degrading the resolution of the device. In addition, there is a need in the art for a compass scale having sufficient contrast with the background on which it is affixed in order to enhance viewing of the scale.